High resolution patterning

The minimum size of the monochrome pixels (we consider color in Section 13.7.3) that can be fabricated using OLEDs is dictated primarily by the ability to pattern the electrode which is deposited on top. OLEDs are not sufficiently robust to withstand the normal processes of photolithography. Among the schemes which have been suggested for high resolution patterning is one in which the substrate is pne-pattemed to provide its own shadow mask [1911. By this means, pixel sizes down to 300 p have been demonstrated, and a lower limit of about 100 p is estimated. 

The fifth of the color methods places the three emitting structures in a stack one on top of the other, rather than side by side ]20l ]. Clearly there is a requirement here that the two electrodes in the middle of the structure must be transparent. The advantages are that the display can be made much brighter with up to three times the luminance from each pixel, and the requirements for high resolution patterning are relaxed by a factor of three. The disadvantages are that three times as many layers must be coated (without defects) over the area of the display and electrical driving circuitry must make contact with four sets of elec- trades. It will be extremely difficult to incorporate a stacked OLED into a active matrix array. 

The function of the interlevel dielectric of the multilevel structure is three-fold (1) it must provide planarization of underlying topography while allowing high resolution patterning of via holes necessary for contact between metal layers, (2) it must provide insulation integrity, and (3) it must contribute minimally to device capacitance. 

The topmost imaging layer can be as thin as possible to obtain the high resolution pattern. 

As shown in Figure 8, a high resolution pattern is obtained by reducing the resist thickness in 1LR system, but in VLSI fabrication process resist thickness more than 1.0 is needed to planarize the topographic surface, so that 2LR system is more useful for submicron fabrication than 1LR system. 

Figure 10.13 shows examples of high-resolution patterns formed on small scale cylindrical objects - optical fiber and microcapillary tubes [1]. These simple devices (integrated photomasks for optical fiber Bragg gratings and intravascular stents), require only one patterned layer. 

Dip-pen nanolithography is a high resolution patterning technique that enables the creation of patterns from the sub lOOnm to many micrometers length scale.76 This technique uses an ink-coated AFM (atomic force microscopy) tip as a nanopen. The ink molecules are transported from the tip to a substrate, normally by capillary forces when the tip is in contact with the surface of the substrate.77 The driving force for such transport is chemisorption of the ink to the underlying substrate due to a chemical78 or electrochemical force.79... 

Adding a monochromator between the soitrce and the sample makes it possible to avoid this problem. However, since the incident intensity is then smaller, the total acquisition time will be longer, often a few days or even np to a week for high resolution patterns. 

M. Colburn et al.. Step and flash imprint lithography A new approach to high-resolution patterning, Proc. SPIE, 3676, 379, 1999. 

Spears and H.I. Smith, High resolution pattern replication using soft X rays, Electron Lett. 8, 102 (1972) E. Spears and H.I. Smith, X ray lithography A new high resolution replication process, Solid State Technol. 15(7), 21 (1972). 

One of the main advantages of DNQ/novolac resists over the bis-azide/rubber resist system that was responsible for the former superseding the latter is the fact that novolac does not swell during development and is therefore suitable for the printing of high-resolution patterns. Additionally, novolac resin is reasonably resistant to plasma, a very important property in semiconductor device fabrication. However, the thermal and mechanical properties of novolac are not outstanding its plastic flow temperature is around 120°C. Even here, the effects of isomer composition... 

---